In recent years, the development of fuel-cell vehicles that run using hydrogen as the fuel and researches on practical hydrogen stations for supplying hydrogen to fuel-cell vehicles have been advanced. A stainless steel is one of candidate materials used for these applications; still, in a high-pressure hydrogen gas environment, the stainless steel may be susceptible to embrittlement caused by hydrogen gas (hydrogen environment embrittlement). In accordance with the Exemplified Standards of Compressed Hydrogen Vehicle Container stipulated in the High Pressure Gas Safety Act, the use of austenitic SUS316L is approved as a stainless steel that is not susceptible to hydrogen embrittlement.
In consideration of the necessity for reduced weight of fuel-cell vehicle and for high-pressure operation of hydrogen station, however, for a stainless steel used for a container and a pipe, there has been a need for stainless steel that has a strength higher than that of the existing SUS316L, especially has a tensile strength of 800 MPa or higher and is not susceptible to hydrogen environment embrittlement in a hydrogen gas environment. That is, assuming the use of high-pressure hydrogen of about 70 MPa, it is estimated that the SUS316L requires a pipe and container to have a wall thickness of 20 mm or larger, which leads to a significant increase in empty vehicle weight, so that higher strength of steel is indispensable.
As a method for enhancing the strength of steel, cold rolling can be cited as a typical method. Patent Document 1 gives a description concerning the cold rolling and the hydrogen environment embrittlement property of austenitic stainless steel.
As means for strengthening the austenitic stainless steel and improving the hydrogen embrittlement property of the austenitic stainless steel without relying on strengthening by cold rolling, Patent Documents 2 and 3 propose high-strength stainless steels for high-pressure hydrogen gas, in which precipitation strengthening by means of fine nitrides is utilized.
Patent Document 2 proposes a high-strength austenitic stainless steel in which 7 to 30% of Mn, 15 to 22% of Cr, and 5 to 20% of Ni are contained as principal components, and Patent Document 3 proposes a high-strength austenitic stainless steel in which 3 to 30% of Mn, more than 22% to 30% or less of Cr, and 17 to 20% of Ni are contained as principal components. These Documents indicate that a tensile strength of 800 MPa or higher can be realized in a state of solid solution heat treatment.